Laser processing method

ABSTRACT

A laser processing method for preventing particles from occurring from cut sections of chips obtained by cutting a silicon wafer is provided. An irradiation condition of laser light L for forming modified regions  7   7  to  7   12  is made different from an irradiation condition of laser light L for forming the modified regions  7   13  to  7   19  such as to correct the spherical aberration of laser light L in areas where the depth from the front face  3  of a silicon wafer  11  is 335 μm to 525 μm. Therefore, even when the silicon wafer  11  and a functional device layer  16  are cut into semiconductor chips from modified regions  7   1  to  7   19  acting as a cutting start point, twist hackles do not appear remarkably in the areas where the depth is 335 μm to 525 μm, whereby particles are hard to occur.

TECHNICAL FIELD

The present invention relates to a laser processing method which forms aplurality of rows of modified regions to become a cutting start pointwithin a silicon wafer along a line to cut the silicon wafer.

BACKGROUND ART

Conventionally known as this kind of technique is a method ofirradiating a semiconductor substrate with laser light so as to form aplurality of rows of modified parts within the semiconductor substratealong a street of the semiconductor substrate, and cutting thesemiconductor substrate along the street (see, for example, PatentDocument 1).

-   Patent Document 1: Japanese Patent Application Laid-Open No.    2005-19667

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

When cutting the semiconductor substrate by the above-mentioned method,however, particles may occur from thus cut section and contaminatesemiconductor chips obtained by the cutting.

In view of such circumstances, it is an object of the present inventionto provide a laser processing method for preventing particles fromoccurring from cut sections of chips obtained by cutting a siliconwafer.

Means for Solving Problem

The inventors conducted diligent studies in order to achieve theabove-mentioned object and, as a result, have found out that particlesoccur from cut sections of chips obtained by cutting a silicon waferbecause of twist hackles appearing in modified regions formed inpredetermined areas of the silicon wafer.

Namely, when laser light is converged within a silicon wafer having athickness t (500 μm<t) by a condenser lens, so as to form and cut aplurality of rows of modified regions within the silicon wafer along aline to cut the silicon wafer, twist hackles remarkably appear on thecut sections in at least areas of the silicon wafer where the depth fromthe laser light entrance surface is 350 μm to 500 μm. Therefore, minuteparts between the twist hackles peel off, thereby generating siliconparticles.

FIG. 18 is a view showing a photograph of a cut section of a siliconwafer when forming 19 rows of modified regions for a line to cut in thesilicon wafer having a thickness of 625 μm by a conventional laserprocessing method. Modified regions 7 ₁ to 7 ₁₉ were successively formedfrom the rear face 21 side under the conditions listed in the followingTable 1, while using the front face 3 of the silicon wafer 11 as thelaser light entrance surface. In the following Table 1, the convergingpoint position refers to the distance from the front face 3 to theposition at which the laser light converging point is focused (ditto inthe following). The exit output is the output of the laser light emittedfrom the condenser lens, while the divergent angle is the divergentangle of laser light incident on the condenser lens (ditto in thefollowing).

TABLE 1 Converging point Exit Divergent position (μm) output (W) angle(°) Modified region 7₁ 618 1.2 0.2 Modified region 7₂ 599 1.2 0.2Modified region 7₃ 580 1.2 0.2 Modified region 7₄ 561 1.2 0.2 Modifiedregion 7₅ 535 1.2 0.2 Modified region 7₆ 517 1.2 0.2 Modified region 7₇494 1.2 0.2 Modified region 7₈ 467 1.2 0.2 Modified region 7₉ 440 1.20.2 Modified region 7₁₀ 409 1.2 0.2 Modified region 7₁₁ 375 1.2 0.2Modified region 7₁₂ 342 1.2 0.2 Modified region 7₁₃ 305 1.2 0.2 Modifiedregion 7₁₄ 280 1.2 0.2 Modified region 7₁₅ 245 1.2 0.2 Modified region7₁₆ 210 1.2 0.2 Modified region 7₁₇ 174 1.2 0.2 Modified region 7₁₈ 1211.2 0.2 Modified region 7₁₉ 69 0.72 0.2

In this example, as shown in FIG. 18, twist hackles (dark parts) 51remarkably appeared in areas of the silicon wafer 11 where the depthfrom the front face 3 was 310 μm to 540 μm. It seems that, when thetwist hackles 51 appear remarkably, minute parts 52 between the twisthackles 51 are likely to peel off, whereby particles are easy to occurin the areas where the depth from the front face 3 was 310 μm to 540 μm.In areas of the silicon wafer 11 where the depth from the front face 3exceeds 540 μm, the twist hackles 51 appear only unremarkably as shownin FIG. 20, so that particles seem to be hard to occur.

Then, the inventors have found that twist hackles remarkably appear oncut sections in at least areas of a silicon wafer where the depth fromthe laser light entrance surface is 350 μm to 500 μm because of anincrease in spherical aberration of laser light in the areas having thedepth of 350 μm to 500 μm.

When incident on the silicon wafer 11, laser light L advances whilebeing refracted according to Snell's law as shown in FIG. 21. Therefore,as a condenser lens 53 is brought closer to the silicon wafer 11 inorder to converge the laser light into an area of the wafer 11 deeperfrom the front face 3, the converging position P1 of center rays L1 andthe converging position P2 of marginal rays L2 deviate more from eachother in the thickness direction of the wafer 11. Consequently, theconverging point of laser light L widens in the thickness direction ofthe wafer 11, so that the degree of convergence of laser light Lworsens, thereby forming a low-quality, uneven modified region extendingin the thickness direction of the wafer 11 (a modified region which issevered into a plurality of parts without connecting together in thethickness direction of the wafer 11 notwithstanding one shot of thelaser light L). As a result, it seems that low-quality modified regionsunnaturally try to connect each other when severed, so as to generatetwist hackles, while uneven fractures occur because of uneven modifiedregions, whereby particles peeling from cut sections of the wafer 11 arelikely to occur between the fractures and twist hackles. The twisthackles 51 do not appear remarkably in areas of the silicon wafer 11where the depth from the front face 3 exceeds 540 μm in theabove-mentioned example because of the following reason. Namely, itseems that the energy density of laser light L at the converging point Pbecomes smaller in the areas having the depth exceeding 540 μm, so thatthe width of the modified region formed in the thickness direction ofthe wafer 11 becomes smaller (which seems to be because a part of thelaser light L having a lower degree of convergence yields such a smallenergy that it fails to exceed a processing threshold), uneven modifiedregions decrease, while reducing uneven fractures at the time ofsevering, whereby twist hackles can be restrained from occurring, andparticles generated between the twist hackles and fractures decrease.

The inventors conducted further studies based on the foregoing findingsand have completed the present invention.

In one aspect, the laser processing method in accordance with thepresent invention is a laser processing method of converging laser lightwithin a silicon wafer having a thickness t (500 μm<t) with a condenserlens so as to form a plurality of rows of modified regions to become acutting start point within the silicon wafer along a line to cut thesilicon wafer, the method comprising the steps of forming a firstmodified region along the line to cut in a first area having a depth of350 μm to 500 μm from a laser light entrance surface of the siliconwafer, and forming a second modified region along the line to cut in asecond area having a depth of 0 μm to 250 μm from the laser lightentrance surface; wherein a laser light irradiation condition forforming the first modified region is made different from a laser lightirradiation condition for forming the second modified region such as tocorrect a spherical aberration of the laser light in the first area.

In another aspect, the laser processing method in accordance with thepresent invention is a laser processing method of converging laser lightwithin a silicon wafer having a thickness t (350 μm<t≦500 μm) with acondenser lens so as to form a plurality of rows of modified regions tobecome a cutting start point within the silicon wafer along a line tocut the silicon wafer, the method comprising the steps of forming afirst modified region along the line to cut in a first area having adepth of 350 μm to t μm from a laser light entrance surface of thesilicon wafer, and forming a second modified region along the line tocut in a second area having a depth of 0 μm to 250 μm from the laserlight entrance surface; wherein a laser light irradiation condition forforming the first modified region is made different from a laser lightirradiation condition for forming the second modified region such as tocorrect a spherical aberration of the laser light in the first area.

In these laser processing methods, laser light irradiation conditionsare changed such that the spherical aberration of laser light iscorrected when forming the first modified region in the first areahaving a depth of 350 μm to 500 μm from the laser light entrance surfaceof the silicon wafer for the silicon wafer having a thickness t (500μm<t) and when forming the first modified region in the first areahaving a depth of 350 μm to t μm from the laser light entrance surfaceof the silicon wafer for the silicon wafer having a thickness t (350μm<t≦500 μm). Therefore, when cutting the silicon wafer into chips fromthe first and second modified regions acting as a cutting start point,twist hackles do not appear remarkably in the first area, wherebyparticles are hard to occur. Hence, these laser cutting methods canprevent particles from occurring from cut sections of chips during andafter cutting. The step of forming the first modified region in thefirst area and the step of forming the second modified region in thesecond area can be performed in any order.

In the laser processing methods in accordance with the presentinvention, the first and second modified regions are formed bygenerating multiphoton absorption or other optical absorptions withinthe silicon wafer by converging the laser light within the silicon waferwith the condenser lens. An example of the first and second modifiedregions formed within the silicon wafer is a molten processed region.

Preferably, in the laser processing methods in accordance with thepresent invention, the laser light incident on the condenser lens whenforming the first modified region has a divergent angle greater thanthat of the laser light incident on the condenser lens when forming thesecond modified region. Preferably, the condenser lens when forming thefirst modified region has an exit NA greater than that of the condenserlens when forming the second modified region. Preferably, a sphericalaberration correction member is arranged between the condenser lens andsilicon wafer when forming the first modified region. These can make thelaser irradiation condition for forming the first modified regiondifferent from the laser irradiation condition for forming the secondmodified region such as to correct the spherical aberration of laserlight in the first region.

Effect of the Invention

Even when a silicon wafer is cut into chips, the present invention canprevent particles from occurring from cut sections of the chips duringand after the cutting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an object to be processed during laserprocessing by the laser processing method in accordance with anembodiment;

FIG. 2 is a sectional view of the object taken along the line II-II ofFIG. 1;

FIG. 3 is a plan view of the object after the laser processing by thelaser processing method in accordance with the embodiment;

FIG. 4 is a sectional view of the object taken along the line IV-IV ofFIG. 3;

FIG. 5 is a sectional view of the object taken along the line V-V ofFIG. 3;

FIG. 6 is a plan view of the object cut by the laser processing methodin accordance with the embodiment;

FIG. 7 is a view showing a photograph of a cut section in a part of asilicon wafer cut by the laser processing method in accordance with theembodiment;

FIG. 8 is a graph showing relationships between the laser lightwavelength and the transmittance within a silicon substrate in the laserprocessing method in accordance with the embodiment;

FIG. 9 is a plan view of the object in the laser processing method inaccordance with the embodiment;

FIG. 10 is a sectional view of a part of the object taken along the lineX-X of FIG. 9;

FIG. 11 is a view for explaining the laser processing method inaccordance with the embodiment, in which (a) is a state where anexpandable tape is attached to the object, and (b) is a state where theobject is irradiated with laser light;

FIG. 12 is a diagram showing a beam expander for changing the divergentangle of laser light incident on a condenser lens;

FIG. 13 is a view for explaining the laser processing method inaccordance with the embodiment, illustrating a state where theexpandable tape is expanded;

FIG. 14 is a view showing a photograph obtained when forming 19 rows ofmodified regions for a line to cut within a silicon wafer having athickness of 625 μm by the laser processing method in accordance withthe embodiment;

FIG. 15 is a diagram showing a condenser lens for changing the exit NA;

FIG. 16 is a diagram showing a condenser lens and spherical aberrationcorrection member for changing a spherical aberration;

FIG. 17 is a diagram showing the beam expander for changing thedivergent angle of laser light incident on the condenser lens and therelationship between the Gaussian distribution of laser light incidenton the condenser lens and the entrance pupil diameter of the condenserlens;

FIG. 18 is a view showing a photograph obtained when foaming 19 rows ofmodified regions for a line to cut within a silicon wafer having athickness of 625 μm by the conventional laser processing method;

FIG. 19 is a schematic view of a cut section in an area of the siliconwafer where the depth from the front face is 310 μm to 540 μm;

FIG. 20 is a schematic view of a cut section in an area of the siliconwafer where the depth from the front face exceeds 540 μm; and

FIG. 21 is a schematic view showing a state of progress of laser lightincident on the silicon wafer.

EXPLANATIONS OF NUMERALS OR LETTERS

3 . . . front face (laser light entrance surface); 5 . . . line to cut;7 . . . modified region; 11 . . . silicon wafer; 13 . . . moltenprocessed region; 53 . . . condenser lens; 57 . . . spherical aberrationcorrection member; L . . . laser light

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, a preferred embodiment of the present invention willbe explained in detail with reference to the drawings. In the laserprocessing method in accordance with the embodiment, a phenomenon knownas multiphoton absorption is used for forming a modified region withinan object to be processed. Therefore, to begin with, a laser processingmethod for forming a modified region by the multiphoton absorption willbe explained.

A material becomes transparent when its absorption bandgap E_(G) isgreater than photon energy hν. Consequently, a condition under whichabsorption occurs in the material is hν>E_(G). However, even whenoptically transparent, the material generates absorption under acondition of nhν>E_(G) (where n=2, 3, 4, . . . ) if the intensity oflaser light becomes very high. This phenomenon is known as multiphotonabsorption. In the case of pulsed waves, the intensity of laser light isdetermined by the peak power density (W/cm²) of laser light at itsconverging point. The multiphoton absorption occurs under a conditionwhere the peak power density is 1×10⁸ (W/cm²) or greater, for example.The peak power density is determined by (energy of laser light at theconverging point per pulse)/(beam spot cross-sectional area of laserlight×pulse width). In the case of continuous waves, the intensity oflaser light is determined by the field intensity (W/cm²) of laser lightat the converging point.

The principle of the laser processing method in accordance with theembodiment using such multiphoton absorption will be explained withreference to FIGS. 1 to 6. As shown in FIG. 1, on a front face 3 of awafer-shaped (planar) object to be processed 1, a line to cut 5 forcutting the object 1 exists. The line to cut 5 is a virtual lineextending straight. As shown in FIG. 2, the laser processing method inaccordance with this embodiment irradiates the object 1 with laser lightL while locating a converging point P therewithin under a conditiongenerating multiphoton absorption, so as to form a modified region 7.The converging point P is a position at which the laser light L isconverged. The line to cut 5 may be curved instead of being straight,and may be a line actually drawn on the object 1 without beingrestricted to the virtual line.

Then, the laser light L is relatively moved along the line to cut 5(i.e., in the direction of arrow A in FIG. 1), so as to shift theconverging point P along the line to cut 5. Consequently, as shown inFIGS. 3 to 5, the modified region 7 is formed along the line to cut 5within the object 1, and becomes a starting point region for cutting 8.The starting point region for cutting 8 refers to a region which becomesa start point for cutting (fracturing) when the object 1 is cut. Thestarting point region for cutting 8 may be made by forming the modifiedregion 7 either continuously or intermittently.

In the laser processing method in accordance with this embodiment, themodified region 7 is not formed by the heat generated from the object 1absorbing the laser light L. The laser light L is transmitted throughthe object 1, so as to generate multiphoton absorption therewithin,thereby forming the modified region 7. Therefore, the front face 3 ofthe object 1 hardly absorbs the laser light L and does not melt.

Forming the starting point region for cutting 8 within the object 1makes it easier to generate fractures from the starting point region forcutting 8 acting as a start point, whereby the object 1 can be cut witha relatively small force as shown in FIG. 6. Therefore, the object 1 canbe cut with a high precision without generating unnecessary fractures onthe front face 3 of the object 1.

There seem to be the following two ways of cutting the object 1 from thestarting point region for cutting 8 acting as a start point. One iswhere an artificial force is applied to the object 1 after the startingpoint region for cutting 8 is formed, so that the object 1 fracturesfrom the starting point region for cutting 8 acting as a start point,whereby the object 1 is cut. This is the cutting in the case where theobject 1 has a large thickness, for example. Applying an artificialforce refers to exerting a bending stress or shear stress on the object1 along the starting point region for cutting 8, or generating a thermalstress by applying a temperature difference to the object 1, forexample. The other is where the forming of the starting point region forcutting 8 causes the object 1 to fracture naturally in itscross-sectional direction (thickness direction) from the starting pointregion for cutting 8 acting as a start point, thereby cutting the object1. This becomes possible if the starting point region for cutting 8 isformed by one row of the modified region 7 when the object 1 has a smallthickness, or if the starting point region for cutting 8 is formed by aplurality of rows of the modified region 7 in the thickness directionwhen the object 1 has a large thickness. Even in this naturallyfracturing case, fractures do not extend onto the front face 3 at aportion corresponding to an area not formed with the starting pointregion for cutting 8 in the part to cut, so that only the portioncorresponding to the area formed with the starting point region forcutting 8 can be cleaved, whereby cleavage can be controlled well. Sucha cleaving method with a favorable controllability is very effective,since the object 1 such as silicon wafer has recently been apt todecrease its thickness.

An example of the modified region formed by multiphoton absorption inthe laser processing method in accordance with the embodiment is amolten processed region.

An object to be processed (e.g., semiconductor material such as silicon)is irradiated with laser light while locating a converging point withinthe object under a condition with a field intensity of at least 1×10⁸(W/cm²) at the converging point and a pulse width of 1 μs or less. As aconsequence, the inside of the object is locally heated by multiphotonabsorption. This heating forms a molten processed region within theobject. The molten processed region encompasses regions once molten andthen re-solidified, regions just in a molten state, and regions in theprocess of being re-solidified from the molten state, and can also bereferred to as a region whose phase has changed or a region whosecrystal structure has changed. The molten processed region may also bereferred to as a region in which a certain structure changes to anotherstructure among monocrystal, amorphous, and polycrystal structures. Forexample, it means a region having changed from the monocrystal structureto the amorphous structure, a region having changed from the monocrystalstructure to the polycrystal structure, or a region having changed fromthe monocrystal structure to a structure containing amorphous andpolycrystal structures. When the object to be processed is of a siliconmonocrystal structure, the molten processed region is an amorphoussilicon structure, for example. The upper limit of field intensity is1×10¹² (W/cm²), for example. The pulse width is preferably 1 ns to 200ns, for example.

By an experiment, the inventors verified that a molten processed regionwas formed within a silicon wafer. The following are conditions of theexperiment.

(A) Object to be processed: silicon wafer (with a thickness of 350 μmand an outer diameter of 4 inches)

(B) Laser

-   -   light source: semiconductor laser pumping Nd:YAG laser    -   wavelength: 1064 nm    -   laser light spot cross-sectional area: 3.14×10⁻⁸ cm²    -   oscillation mode: Q-switched pulse    -   repetition frequency: 100 kHz    -   pulse width: 30 ns    -   output: 20 μJ/pulse    -   laser light quality: TEM₀₀    -   polarizing property: linear polarization

(C) Condenser lens

-   -   magnification: ×50    -   N.A.: 0.55    -   transmittance at a laser light wavelength: 60%

(D) Moving rate of the mount table mounting the object: 100 mm/sec

FIG. 7 is a view showing a photograph of a cross section of a part of asilicon wafer cut by laser processing under the conditions mentionedabove. A molten processed region 13 is formed within the silicon wafer11. The molten processed region 13 formed under the above-mentionedconditions has a size of about 100 μm in the thickness direction.

The fact that the molten processed region 13 is formed by multiphotonabsorption will now be explained. FIG. 8 is a graph showingrelationships between the laser light wavelength and the transmittancewithin the silicon substrate. Here, the respective reflected componentson the front and rear face sides of the silicon substrate areeliminated, so as to show the internal transmittance alone. Therespective relationships are shown in the cases where the thickness t ofthe silicon substrate is 50 μm, 100 μm, 200 μm, 500 μm, and 1000 μm.

For example, at the Nd:YAG laser wavelength of 1064 nm, the laser lightappears to be transmitted through the silicon substrate by at least 80%when the silicon substrate has a thickness of 500 μm or less. Since thesilicon wafer 11 shown in FIG. 7 has a thickness of 350 μm, the moltenprocessed region 13 caused by multiphoton absorption is formed near thecenter of the silicon wafer 11, i.e., at a part distanced from the frontface by 175 μm. The transmittance in this case is 90% or more withreference to a silicon wafer having a thickness of 200 μm, whereby thelaser light is absorbed only slightly within the silicon wafer 11 but issubstantially transmitted therethrough. This means that the moltenprocessed region 13 is formed within the silicon wafer 11 not by laserlight absorption within the silicon wafer 11 (i.e., not by usual heatingwith the laser light) but by multiphoton absorption. The forming of amolten processed region by multiphoton absorption is disclosed, forexample, in “Silicon Processing Characteristic Evaluation by PicosecondPulse Laser”, Preprints of the National Meetings of Japan WeldingSociety, Vol. 66 (April, 2000), pp. 72-73.

A fracture is generated in a silicon wafer from a starting point regionfor cutting formed by a molten processed region, acting as a startpoint, in a cross-sectional direction, and reaches the front and rearfaces of the silicon wafer, whereby the silicon wafer is cut. Thefracture reaching the front and rear faces of the silicon wafer may grownaturally or as a force is applied to the silicon wafer. The fracturenaturally growing from the starting point region for cutting to thefront and rear faces of the silicon wafer encompasses a case where thefracture grows from a state where the molten processed region formingthe starting point region for cutting is molten and a case where thefracture grows when the molten processed region forming the startingpoint region for cutting is re-solidified from the molten state. Ineither case, the molten processed region is formed only within thesilicon wafer, and thus is present only within the cut section aftercutting as shown in FIG. 7. When a starting point region for cutting isthus formed within the object by a molten processed region, unnecessaryfractures deviating from a starting point region for cutting line areharder to occur at the time of cleaving, whereby cleavage controlbecomes easier.

While the molten processed region is explained in the foregoing as amodified region formed by multiphoton absorption, a starting pointregion for cutting may be formed as follows while taking account of thecrystal structure of a wafer-like object to be processed, its cleavagecharacteristic, and the like, whereby the object can be cut with afavorable precision by a smaller force from the starting point regionfor cutting acting as a start point.

Namely, in the case of a substrate made of a monocrystal semiconductorhaving a diamond structure such as silicon, it will be preferred if astarting point region for cutting is formed in a direction extendingalong a (111) plane (first cleavage plane) or a (110) plane (secondcleavage plane). In the case of a substrate made of a group III-Vcompound semiconductor of sphalerite structure such as GaAs, it will bepreferred if a starting point region for cutting is formed in adirection extending along a (110) plane. In the case of a substratehaving a crystal structure of hexagonal system such as sapphire (Al₂O₃),it will be preferred if a starting point region for cutting is formed ina direction extending along a (1120) plane (A plane) or a (1100) plane(M plane) while using a (0001) plane (C plane) as a principal plane.

When the substrate is formed with an orientation flat in a direction tobe formed with the above-mentioned starting point region for cutting(e.g., a direction extending along a (111) plane in a monocrystalsilicon substrate) or a direction orthogonal to the direction to beformed therewith, the starting point region for cutting extending in thedirection to be formed with the starting point region for cutting can beformed easily and accurately in the substrate with reference to theorientation flat.

The preferred embodiment of the present invention will now be explained.FIG. 9 is a plan view of the object to be processed in the laserprocessing method in accordance with the embodiment, whereas FIG. 10 isa sectional view of a part of the object taken along the line X-X ofFIG. 9.

As shown in FIGS. 9 and 10, the object 1 comprises a silicon wafer 11having a thickness of 625 μm and a functional device layer 16, formed onthe front face 3 of the silicon wafer 11, including a plurality offunctional devices 15. A number of functional devices 15, examples ofwhich include semiconductor operating layers formed by crystal growth,light-receiving devices such as photodiodes, light-emitting devices suchas laser diodes, and circuit devices formed as circuits, are formed likea matrix in directions parallel and perpendicular to an orientation flat6 of the silicon wafer 11.

Thus constructed object 1 is cut into the functional devices 15 asfollows. First, an expandable tape 23 is attached to the rear face 21 ofthe silicon wafer 11 as shown in FIG. 11( a). Subsequently, as shown inFIG. 11( b), the object 1 is secured onto a mount table of a laserprocessing apparatus (not shown) with the functional device layer 16facing up.

Then, the object 1 is irradiated with laser light L, while using thefront face 3 as a laser light entrance surface and locating a convergingpoint P within the silicon wafer 11 under a condition causingmultiphoton absorption, and the mount table is moved such as to scan theconverging point P along lines to cut 5 (see broken lines in FIG. 9)which are set like grids passing between the functional devices 15, 15adjacent to each other.

The scanning of the converging point P along the lines to cut 5 isperformed 19 times for each line to cut 5 with respective distances fromthe front face 3 to the position at which the converging point P isfocused, whereby 19 rows of modified regions 7 ₁ to 7 ₁₉ aresuccessively formed one by one from the rear face 21 side within thesilicon wafer 11 along the line to cut 5 under the conditions shown inthe following Table 2. Each of the modified regions 7 ₁ to 7 ₁₉ is amolten processed region which may include cracks mixed therein.

TABLE 2 Converging point Exit Divergent position (μm) output (W) angle(°) Modified region 7₁ 618 1.2 0.2 Modified region 7₂ 603 1.2 0.2Modified region 7₃ 588 1.2 0.2 Modified region 7₄ 573 1.2 0.2 Modifiedregion 7₅ 558 1.2 0.2 Modified region 7₆ 545 1.2 0.2 Modified region 7₇520 0.92 0.4 Modified region 7₈ 496 0.92 0.4 Modified region 7₉ 469 0.920.4 Modified region 7₁₀ 434 0.92 0.4 Modified region 7₁₁ 404 0.92 0.4Modified region 7₁₂ 368 0.92 0.4 Modified region 7₁₃ 330 1.2 0.2Modified region 7₁₄ 299 1.2 0.2 Modified region 7₁₅ 261 1.2 0.2 Modifiedregion 7₁₆ 220 1.2 0.2 Modified region 7₁₇ 175 1.2 0.2 Modified region7₁₈ 120 1.2 0.2 Modified region 7₁₉ 73 0.72 0.2

As can be seen from the above-mentioned Table 2, the divergent angle ofthe laser light L incident on the condenser lens of the laser processingapparatus is set to 0.4° when forming the modified regions 7 ₇ to 7 ₁₂positioned in areas where the depth from the front face 3 of the siliconwafer 11 is 335 μm to 525 μm. On the other hand, the divergent angle ofthe laser light L incident on the condenser lens of the laser processingapparatus is set to 0.2° when forming the modified regions 7 ₁ to 7 ₆positioned in areas where the depth from the front face 3 of the siliconwafer 11 exceeds 525 μm and when forming the modified regions 7 ₁₃ to 7₁₉ positioned in areas where the depth is less than 335 μm. Forincreasing the divergent angle of the laser light L incident on thecondenser lens, it will be sufficient if a convex lens 55 is broughtcloser to a concave lens 56 in a beam expander 54 as shown in FIGS. 12(a) and (b).

After forming the modified regions 7 ₁₃ to 7 ₁₉, the expandable tape 23is expanded as shown in FIG. 13, so as to generate a fracture from eachof the modified regions 7 ₁₃ to 7 ₁₉ acting as a start point, therebycutting the silicon wafer 11 and functional device layer 16 along thelines to cut 5 and separating semiconductor chips 25 obtained by thecutting from each other.

In the above-mentioned laser processing method, as explained in theforegoing, the divergent angle (0.4°) of the laser light L incident onthe condenser lens when forming the modified regions 7 ₇ to 7 ₁₂ is madegreater than the divergent angle (0.2°) of the laser light L incident onthe condenser lens when forming the modified regions 7 ₁₃ to 7 ₁₉. Thismakes the irradiation condition of laser light L for forming themodified regions 7 ₇ to 7 ₁₂ different from the irradiation condition oflaser light L for forming the modified regions 7 ₁₃ to 7 ₁₉ such as tocorrect the spherical aberration of laser light L in the areas where thedepth from the front face 3 of the silicon wafer 11 is 335 μm to 525 μm.Therefore, even when the silicon wafer 11 and functional device layer 16are cut into the semiconductor chips 25 from the modified regions 7 ₁ to7 ₁₉ acting as a cutting start point, twist hackles do not appearremarkably in the areas where the depth is 335 μm to 525 μm, wherebyparticles are hard to occur. Hence, the above-mentioned laser processingmethod can prevent particles from occurring from cut sections of thesemiconductor chips 25.

FIG. 14 is a view showing a photograph obtained when forming 19 rows ofmodified regions for a line to cut within a silicon wafer having athickness of 625 μm by the laser processing method in accordance withthe embodiment. In this example, as shown in the drawing, twist hacklesdo not appear remarkably in the areas where the depth from the frontface 3 of the silicon wafer 11 is 335 μm to 525 μm in addition to theareas where the depth is more than 525 μm and less than 335 μm.Therefore, the cut sections of the silicon wafer 11 seem to be in astate where particles are hard to occur.

Meanwhile, when the divergent angle of the laser light L incident on thecondenser lens of the laser processing apparatus is set to 0.4° in theareas where the depth from the front face 3 of the silicon wafer 11exceeds 525 μm as in the areas where the depth is 335 μm to 525 μm, aproblem of the modified regions 7 ₁ to 7 ₆ failing to connect togetheroccurs because of energy shortage in the laser light L. For solving thisproblem, the energy of laser light L may be enhanced without setting thedivergent angle of the laser light L incident on the condenser lens to0.2°.

When the divergent angle of the laser light L incident on the condenserlens of the laser processing apparatus is set to 0.4° in the areas wherethe depth from the front face 3 of the silicon wafer 11 is less than 335μm as in the areas where the depth is 335 μm to 525 μm, a problem ofincreasing twist hackles and the like occur. Setting the divergent angleof the laser light L incident on the condenser lens to 0.2° is effectivein solving this problem.

The present invention is not limited to the above-mentioned embodiment.

For example, the divergent angle of the laser light L incident on thecondenser lens when forming the modified regions 7 ₇ to 7 ₁₂ may be madenot only greater than that of the laser light L incident on thecondenser lens when forming the modified regions 7 ₁₃ to 7 ₁₉ but alsoas follows, so as to make the irradiation condition of laser light L forforming the modified regions 7 ₇ to 7 ₁₂ different from the irradiationcondition of laser light L for forming the modified regions 7 ₁₃ to 7 ₁₉such as to correct the spherical aberration of laser light L.

Namely, the exit NA (numerical aperture) of the condenser lens 53 forforming the modified regions 7 ₇ to 7 ₁₂ (see FIG. 12( b)) may be madegreater than the exit NA (numerical aperture) of the condenser lens 53for forming the modified regions 7 ₁₃ to 7 ₁₉ (see FIG. 12( a)).

A spherical aberration correction member 57 may be arranged between thecondenser lens 53 and the silicon wafer when forming the modifiedregions 7 ₇ to 7 ₁₂ (see FIG. 13( b)) but not when forming the modifiedregions 7 ₁₃ to 7 ₁₉ (see FIG. 13( a)). The spherical aberrationcorrection member 57 is a member such as a silica glass sheet having athickness of 0.5 mm, for example, which generates a spherical aberrationin a direction canceling a spherical aberration corresponding to a depthfrom the laser light entrance surface of the silicon wafer.

Also, a condenser lens originally left with such a spherical aberrationas to cancel a spherical aberration corresponding to a depth from thelaser light entrance surface of the silicon wafer may be prepared, andsuch condenser lenses may be switched according to the depth from thelaser light entrance surface of the silicon wafer. Further, a condenserlens which can change a lens performance such as to cancel a sphericalaberration corresponding to a depth from the laser light entrancesurface of the silicon wafer by moving a correction tube or the likewithin the condenser lens may be prepared, so as to change the lensperformance of the condenser lens according to the depth from the laserlight entrance surface of the silicon wafer.

Reducing the influence of spherical aberration instead of correcting thespherical aberration can also restrain twist hackles from appearing.

As the condenser lens is brought closer to the silicon wafer in order toconverge the laser light into an area of the silicon wafer deeper fromthe laser light entrance surface, the converging position of center raysof the laser light and the converging position of marginal rays of thelaser light deviate more from each other in the thickness direction ofthe wafer, so as to widen the energy distribution, whereby twist hacklesappear when cutting a section. Therefore, making the energy of marginalrays lower than a processing threshold can reduce the influence ofmarginal rays, thereby restraining twist hackles from appearing.

For example, when forming the modified regions 7 ₁₃ to 7 ₁₉, thedivergent angle of laser light L is increased by the beam expander 54 asshown in FIG. 17( a), so that the energy of marginal rays incident onthe entrance pupil (with a pupil diameter of d) is higher than theprocessing threshold. When forming the modified regions 7 ₇ to 7 ₁₂, onthe other hand, the divergent angle of laser light L is reduced by thebeam expander 54 as shown in FIG. 17( b), so that the energy of marginalrays incident on the entrance pupil (with the pupil diameter of d) islower than the processing threshold.

While the above-mentioned embodiment is a case where a plurality of rowsof modified regions are formed within a silicon wafer having a thicknesst (500 μm<t) along a line to cut, the following can prevent particlesfrom occurring from cut sections of chips obtained by cutting a siliconwafer having a thickness t (350 μm<t≦500 μm) when forming a plurality ofrows of modified regions within this silicon wafer along a line to cut.Namely, it will be sufficient if the laser light irradiation conditionfor forming a modified region in an area where the depth from the laserlight entrance surface of the silicon wafer is 350 μm to t μm is madedifferent from the laser light irradiation condition for forming amodified region in an area where the depth is 0 μm to 250 μm such as tocorrect the spherical aberration of laser light in the area where thedepth is 350 μm to t μm.

Though the above-mentioned embodiment is a case where the front face ofthe silicon wafer is used as the laser light entrance surface, the rearface of the silicon wafer may be employed as the laser light entrancesurface. Though the above-mentioned embodiment is a case where thefunctional layer exists on lines to cut, the front face of the siliconwafer may be used as the laser light entrance surface in a state wherethe front face of the silicon wafer is exposed with no functional devicelayer on the lines to cut.

The number of rows of modified regions formed within the silicon waferfor a line to cut varies depending on the thickness of the silicon waferand the like without being restricted to 19.

INDUSTRIAL APPLICABILITY

Even when a silicon wafer is cut into chips, the present invention canprevent particles from occurring from cut sections of the chips duringand after the cutting.

1-10. (canceled)
 11. A laser processing method of converging laser lightwithin a silicon wafer having a thickness t (500 μm<t) with a condenserlens so as to form a plurality of rows of modified regions to become acutting start point within the silicon wafer along a line to cut thesilicon wafer, the method comprising the steps of: forming a firstmodified region along the line to cut in a first area having a depth of350 μm to 500 μm from a laser light entrance surface of the siliconwafer by locating a converging point of the laser light in the firstarea; and forming a second modified region along the line to cut in asecond area having a depth of 0 μm to 250 μm from the laser lightentrance surface by locating the converging point in the second area;wherein, when forming the first modified region, the laser light isconverged within the silicon wafer so as to cancel a sphericalaberration to occur in response to a depth from the laser light entrancesurface, and thereby an influence of the spherical aberration isreduced.
 12. A laser processing method of converging laser light withina silicon wafer having a thickness t (350 μm<t≦500 μm) with a condenserlens so as to form a plurality of rows of modified regions to become acutting start point within the silicon wafer along a line to cut thesilicon wafer, the method comprising the steps of: forming a firstmodified region along the line to cut in a first area having a depth of350 μm to t μm from a laser light entrance surface of the silicon waferby locating a converging point of the laser light in the first area; andforming a second modified region along the line to cut in a second areahaving a depth of 0 μm to 250 μm from the laser light entrance surfaceby locating the converging point in the second area; wherein, whenforming the first modified region, the laser light is converged withinthe silicon wafer so as to cancel a spherical aberration to occur inresponse to a depth from the laser light entrance surface, and therebyan influence of the spherical aberration is reduced.
 13. A laserprocessing method according to claim 11, wherein, when forming the firstmodified region, a member which acts to cancel the spherical aberrationto occur in response to the depth from the laser light entrance surfaceis arranged between the condenser lens and the silicon wafer, andthereby the influence of the spherical aberration is reduced.
 14. Alaser processing method according to claim 11, wherein, when forming thefirst modified region, the condenser lens which can change a lensperformance so as to cancel the spherical aberration to occur inresponse to the depth from the laser light entrance surface is used, andthereby the influence of the spherical aberration is reduced.
 15. Alaser processing method according to claim 11, wherein the first andsecond modified regions are molten processed regions.
 16. A laserprocessing method according to claim 12, wherein, when forming the firstmodified region, a member which acts to cancel the spherical aberrationto occur in response to the depth from the laser light entrance surfaceis arranged between the condenser lens and the silicon wafer, andthereby the influence of the spherical aberration is reduced.
 17. Alaser processing method according to claim 12, wherein, when forming thefirst modified region, the condenser lens which can change a lensperformance so as to cancel the spherical aberration to occur inresponse to the depth from the laser light entrance surface is used, andthereby the influence of the spherical aberration is reduced.
 18. Alaser processing method according to claim 12, wherein the first andsecond modified regions are molten processed regions.